There are many contexts in which the ability to assay mammalian blood plasma or serum is extremely important. Firstly, the diagnosis and/or monitoring of many pathological conditions requires such an ability, as does the monitoring of certain pharmaceutical regimens. Secondly, in human beings, such an ability may be important to permit a well-founded assessment of a person's fitness to undertake certain forms of intensive physical activity, such as certain forms of athletic activity. In some of these instances, assay of a single sample of plasma or serum for a given constituent, e.g., a ligand, may provide the desired information; in others, it is important that a series of plasma or serum samples collected at stated intervals or after defined events, such as levels of heart-rate raising exercise or intake of food or medicine, may need to be assayed to provide the desired information.
At present, there are well established means for the collection of whole blood samples and for transport of these samples to a laboratory, followed by processing to separate the red cells from the plasma and analysis of the blood plasma portion thereof to make a qualitative or quantitative assessment as to one or more actual or suspected components thereof. For example, the collection of venous blood in sterile vacuum tubes such as Vacutainer.RTM. brand tubes is well known. But such blood collection requires the services of a trained health professional, such as a nurse, doctor or professional phlebotomist. It is often extremely inconvenient for a mammalian patient to attend at a blood collection site, as well as costly. When it is necessary to collect a series of blood samples from the same subject over intervals of time, moreover, inconvenience and cost are greatly multiplied.
In addition, samples of liquid blood, plasma or serum usually require refrigeration and expeditious transport to the analytical laboratory if their integrity is to be maintained. Despite a variety of chemical stabilizers available in collection tubes such as Vacutainer.RTM. tubes, sample stabilities often are only about 48 hours and commonly this storage time is achieved only when the tubes are kept under refrigeration.
Numerous simple devices are also in use for separation of red blood cells from plasma or serum immediately after the collection of a whole blood sample from a mammalian subject. These devices generally operate on a lateral flow chromatographic principle and are so designed that the separated plasma or serum is immediately subjected to a qualitative or quantitative assay for at least one ligand, often by use of a "throwaway", one-use-only, device pre-impregnated with a binding partner for each ligand to be assayed. Such devices typically provide for lateral flow of the sample along a pre-impregnated pathway and for development of a color reaction when the ligand assayed for is present. In some of these cases the blood separation device and subsequent plasma or serum assay device are constructed as a single disposable unit--but in all such cases the object is to obtain a plasma or serum sample and assay that sample while the mammalian patient is present in the medical practitioner's office. Descriptions of blood separation devices of the lateral flow, chromatographic genre appear, inter alia, in U.S. Pat. Nos. 5,135,719; 4,816,224; 4,477,575; 5,186,843; 5,262,067; 4,933,092 and European patent 0295526. In these and similar known blood separation devices, media used in the separation step include various fiberglasses, composite products of the types, e.g., sold under the registered U.S. trademarks Hemasep and Cytosep, and other hydrophilic fibrous materials having effective pore sizes slightly larger than the hydrodynamic volume of a red cell. In general, the physical dimensions and arrangement of the fibers in these media are such that they impede the flow of red blood cells at the surface to which the blood sample is applied while allowing relatively unimpeded flow of the plasma or serum by capillarity. While the red cells are able to move through the fibrous matrix, their flow rate is much slower than that of the plasma or serum, resulting in the formation of a red-cell-free zone at the leading edge of the flow. Because the plasma or serum volumes supplied by separations effected with these devices are somewhat variable, quantitative assays for ligands using such plasma volumes require the development and use of a calibration curve for each specific ligand being determined. Because such calibration curve development is time-consuming and tedious, and somewhat impractical of performance in many point-of-care milieus, such as medical practitioners' offices, the serum or plasma volume separated by these devices is, as a practical matter, most often subjected to a qualitative assay for the presence of the target ligand.
Simple devices are also known that permit blood sample collection, stabilization and transport, such as depositing a drop of peripheral blood onto a sheet of filter paper and allowing it to dry. Such a device is currently used for qualitative detection of inherited diseases in newborns, such as phenylketonuria. More recently, a similar "test card" comprising filter paper has been suggested for a commercial, mail-in system for the qualitative detection of HIV antibodies in whole blood. Thus, U.S. Pat. No. 5,641,682 refers to a test card of this type supplied in a commercial over-the-counter kit. The person desiring an HIV test is directed to place multiple whole blood spots on the card, allow them to dry, and then to convey the card containing dried blood spots to a laboratory where an assay for HIV antibody is performed. This simpler means of whole blood collection does not require the services of a trained phlebotomist. It suffers from two main disadvantages, however: first, it does not allow for the collection of a defined volume of blood, blood plasma or serum; and second, mere drying of whole blood may not adequately stabilize the blood constituents from the time of collection to the time of assay. Quantitative assays for various substances in plasma or serum, such as antibodies, enzymes, hormones, drugs and small molecular weight constituents such as glucose, cholesterol, or lactic acid, require a knowledge of the plasma or serum sample volume. Dried whole blood samples contain an unknown volume of blood plasma or serum. The plasma or serum volume contained per unit area of a dried blood spot varies according to the hematocrit of the sample and as a result of the duration and other conditions of the drying process; the plasma or serum volumes resulting from lateral chromatographic separation of red cells and plasma have been found to be highly variable. Other sources of variation can be related to the uniformity or lack thereof, and other characteristics of the collection matrix, among other factors.